The T cell antigen receptor (TCR) initiates T cell responses by inducing signal transduction pathways. It does so by directly interacting with two families of protein tyrosine kinases (PTKs), members of the Src and Syk families. Biochemical, structural and genetic studies have helped to define the roles of these PTKs in developmental decisions mediated by the pre- TCR and developmental decisions and activation responses mediated by the mature TCR. Mouse models have played a major role in defining the functions of the Src and Syk families of PTKs involved in signaling pathways mediated by the pre-TCR and mature TCR. In addition, these mice have been used as models of human immunodeficiency syndromes. Although studying the mouse immune system has experimental advantages, it is increasingly clear that the mouse immune system does not perfectly recapitulate the human immune system. In studies of the function of ZAP-70, a Syk family PTK, differences in the T cell compartments of mice and humans deficient in ZAP-70 are quite striking. Mice made deficient in ZAP-70 have a complete developmental arrest in the transition from double positive (DP; CD4+/CD8+) to single positive (SP; CD4+ or CD8+) thymocytes, with no detectable peripheral alphabeta TCR-expressing cells. In contrast, human patients with ZAP-70 deficiencies have a severe combined immunodeficiency (SCID) syndrome characterized by the presence of normal to elevated numbers of peripheral alphabeta TCR-expressing CD4+ T cells, but no peripheral CD8+ T cells. Strikingly, the CD4+ T cells in these patients have a severe TCR signaling defect. The ability of the related Syk PTK to substitute for ZAP- 70 in several systems, led to our hypothesis that Syk might be able to partially compensate for the loss of ZAP-70 in these patients and that Syk expression might be regulated differently in the murine and human T cell lineage. To study this, we developed anti-Sy monoclonal antibodies (mAbs) that could be used to stain Syk intracellularly. Using these mAbs, we have defined the expression of Syk in murine and human thymic subsets These results suggests a difference in expression of Syk in DP thymocytes. Moreover, using these mAbs we have detected a population of Syk-expressing T cells in murine and human and contrast the functions of Syk in human and murine alphabeta TCR T cell lineages and to extend such comparative analyses to other cytoplasmic PTKs involved in TCR signal transduction. We will: 1) Define and compare the mechanisms responsible for regulating Syk expression in human and murine thymocytes; 2) Define the function of Syk in peripheral alphabeta-TCR expressing T cells; and 3) Compare the expression of the Lck, Fyn, CAP- 70 and the Tec family PTKs in human and murine T cell and NK cell subsets.